We estimate that a typical home needs between 20 and 24 solar panels to cover 100 percent of its electricity usage. The actual formula to find out how many solar panels you need can be found by the system size divided by the production ratio, divided by panel wattage.
The actual number you’ll need to install depends on factors including geographic location, panel efficiency, panel rated power, and your personal energy consumption habits. Importantly, the number of solar panels you need for your home directly impacts the price you pay for solar. While the answer isn’t always simple, we’ve put together some example cases to help you understand, at a high level, how many solar panels you need to install an effective solar array.
Annual electricity usage
Your annual electricity usage is the amount of energy and electricity you use in your home over a full year. Measured in kilowatt-hours (kWh), this number is influenced by the appliances in your home that use electricity and how often you use them. Refrigerators, air conditioning units, small kitchen appliances, lights, chargers, and more all use electricity.
According to the U.S. Energy Information Administration (EIA), the average American household uses 10,715 kWh of electricity per year, so we’ll use that number as the ideal solar panel system or solar array size, which would mean you could offset 100 percent of your electricity usage and utility bill with solar panels (in practice, it’s not this neat, but bear with us here). If you’re interested in getting a more exact number based on your personal energy usage, check last year’s utility bills to find out how much electricity you used. Once you have that number, feel free to plug it into the equations below.
Solar panel wattage
Also known as a solar panel’s power rating, panel wattage is the electricity output of a specific solar panel under ideal conditions. Wattage is measured in watts (W), and most solar panels fall in the range of 250 – 400 W of power.
Production ratios
A solar panel system’s production ratio is the ratio of the estimated energy output of a system over time (in kWh) to the actual system size (in W). These numbers are almost never 1:1 – depending on how much sunlight your system will get (which is primarily based on your geographic location), your production ratio will change accordingly.
For example, a 10 kW system that produces 14 kWh of electricity in a year has a production ratio of 1.4 (14/10 = 1.4) – this is an entirely realistic production ratio to see out in the real world. In the U.S., production ratios are usually between 1.3 and 1.6, so we’ll use those two numbers as the high and low estimates for our calculation.
Now, let’s work out the math
We have our three main assumptions (energy use, solar panel wattage, and production ratios) – now how do those numbers translate to an estimated number of solar panels for your home? The formula looks like this:
Number of panels = system size / production ratio / panel wattage
Plugging our numbers in from above, we get:
Number of panels = 10,715 kW / 1.3 or 1.6 / 340 W
…which gives us between 20 and 24 panels in a solar array, depending on which production ratio we use (20 for a 1.6 ratio, and 24 for a 1.3 ratio). So, 20 panels at 340 W each result in a total system size of about 6.8 kW!
Main considerations to determine how much solar you need
Calculating how many solar panels you need can be done with the three inputs above, but digging deeper, there are many more factors at play in determining your ideal solar panel system size. Understanding production ratios, the relationship between system size and the number of panels, and the impact of home size can all be important when you’re designing your ideal solar panel setup.
How many kWh can your solar panels produce?
The amount of power (kWh) your solar energy system can produce depends on how much sunlight exposure your roof receives, which in turn creates your production ratio. The amount of sunlight you get in a year depends on both where you are in the country, and what time of year it is. For instance, California has more sunny days annually than New England. But in either location, you’ll be able to produce enough power to cover your energy needs and say goodbye to your utility bills–if you live in an area that gets less peak sunlight hours, you’ll just need to have a larger solar array system installed at your home. Thus, production ratios differ according to geographic location and a lower production ratio (because of less sunlight) means you’ll need more solar panels to get the amount of energy production you need.
Here’s an example: two comparably sized households in California and Massachusetts consume the average amount of electricity for an American household, which is 10,715 kWh annually as mentioned above. The California household needs about a 6.8 kW system to cover 100 percent of their energy needs. By comparison, the comparable household in Massachusetts needs about an 8.2 kW system to cover their energy needs. Solar panel systems in California are smaller than solar panel systems in Massachusetts but are able to produce the same amount of power because they’re exposed to more peak sunlight hours each year. Homeowners in less sunny areas, like Massachusetts, can make up for this disparity by simply using more efficient panels or increasing the size of their solar energy system, resulting in slightly more solar panels on their rooftop.
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